The present invention relates to aluminum smelting and, more particularly, to an improved aluminum reduction cell for recovering aluminum from Al.sub.2 O.sub.3.
In the production of aluminum by the Hall process, direct current is passed through an electrolyte containing dissolved alumina. The molten electrolyte at a temperature of about 960.degree. C. is contained within a steel shell, the bottom and sides of which are lined with carbonaceous material. Carbon anodes immersed in the molten electrolyte cover much of the surface of the electrolyte. The remainder of the surface is covered by a crust of alumina and frozen electrolyte.
The power required to convert alumina to aluminum amounts to about 21/2 KWH per pound of aluminum. However, the electrical resistance of the electrolyte, the anode, the cathode and interconnecting conductors requires an additional 31/2-41/2 KWH/#. The extra power so supplied is transformed into heat which must be dissipated. The temperature of the electrolyte must be held as closely as possible to optimum--lower temperatures endangering freezing and cessation of operations--higher temperatures resulting in drastic reduction in production efficiencies. Thus a controlled emission of the heat being generated is essential to good operation.
As it is designed and operated, the conventional modern cell reflects an outmoded method of batch feeding the alumina and the outdated assumption of cheap energy. It was originally considered necessary to place the charge of alumina on the surface of the pot several hours before mixing it into the electrolyte in order to preheat it. This resulted in the formation on the surface of the electrolyte a crust which served to restrict the loss of heat and the emission of fluorides. A degree of control was afforded to the pot operator in that he could vary the thickness of the crust, the frequency of breaking it, and even the length of time the molten electrolyte was left exposed before fresh alumina was piled on. Undesirable features were the unmeasured variations introduced by these deliberate changes to say nothing of those from variations in the insulating qualities of alumina. Another variable is that the crust may supply a little or a lot of alumina to the electrolyte between scheduled feeding time. And finally, it is difficult to get a continuous temperature reading of the electrolyte for control purposes. The molten electrolyte is too corrosive to permit continuous immersion of a thermocouple and the crust inhibits a visual observation from about. All this contributes to the difficulty of automating the operation and explains some of the need for artistry in the operation.
The modern concept of feeding alumina is by continuous addition--by passing the preheating on the pot surface. A feeder repeatedly breaks a hole in the crust and alumina is dropped on to the exposed surface of the molten electrolyte. Thus, the crust has lost some of its purpose but continues to function variably in other aspects. In an apparatus described in U.S. Pat. No. 3,951,763, a cover is placed over the pot to contain the heat and to keep the upper surface of the bath in a molten condition. Alumina is continuously fed through the cover. In other respects, however, the pot or cell is more or less conventional.
To complete the picture, the walls and bottom of the conventional pot are designed to dissipate the heat which is not emitted through the surface. The bottom is reasonably well insulated although the collector bars carrying current from the bottom are good radiators of heat. However, the side and end walls are lightly insulated and the shell temperature reaches some 200.degree. C. during operation.
The cell is thus designed to dissipate a specific quantity of heat--with a variation of some 10 percent possible through adjustment of the crust. With a reliable and continuous supply of power, this has proved to be a workable arrangement. Nevertheless, in case of a power interruption, the affected cells can be expected to freeze up in a few hours. If the power supply is reduced, the power requirements of operating cells can be reduced by some 10 percent--and any power shortage beyond that must be covered by letting the surplus cells freeze. The cost of repairing and restarting frozen cells is very high so that the fixed operating level is a real disadvantage when power is not firm. Thus the cells must be designed to operate over a relatively narrow range of available power inputs and even at normal power inputs a great deal of power is simply wasted in the form of dissipated resistive heating.
It may also be noted that although the crust restricts the emissions of fluorides from the surface of the electrolyte, it does not arrest them adequately. It has been necessary to install hoods over the surface to capture the gases produced by electrolysis and other particulate emissions. The vacuum applied to the hoods is intended to ensure a substantial inflow of air through the joints of the hoods so that collection of the pot emissions will be as perfect as possible. The hood flow is passed through bag filters and it is necessary that the temperature be low enough that it does not burn the fabric in the bags.